This proposal outlines studies to characterize the biosynthesis of the mononuclear phagocyte colony-stimulating factor CSF-1 (or M-CSF) and the interaction of this homodimeric growth factor with its receptor. The primary translation products of CSF-1 mRNAs are integral transmembrane proteins from which the soluble growth factor is derived by proteolytic cleavage. The products of two different human CSF-1 cDNAs will be compared with respect to the subcellular location and specificity of proteolytic processing. Site-directed mutagenesis will be used to define the extent of polypeptide sequences required for CSF-1 function and to identify cysteine residues that participate in disulfide bonds critical for dimer formation and biological activity. Membrane-bound CSF-1 molecules that fail to undergo proteolytic processing will be engineered and used to determine whether cell surface CSF-1 can directly stimulate receptor- bearing cells. The topological requirements of ligand-receptor interactions to induce autocrine transformation of cells simultaneously expressing CSF-1 and its receptor will also be investigated. These studies will be general interest to the biosynthesis of polypeptide growth factors and the mechanisms of growth factor-receptor interactions. A possible role for CSF-1 and its receptor outside the context of hematopoiesis will be examined by immunocytochemical techniques and in situ hybridization with specific molecular probes. An intensive study of the structure and expression of this growth factor is merited by the prospects for use of CSF-1 as a biological response modifier to stimulate macrophage production of function in clinical therapeutics.